tests: Removed test-progs/insttest

The insttests source is now found in gem5-resources:
https://gem5.googlesource.com/public/gem5-resources/+/refs/heads/master/src/insttest/

The pre-compiled binaries are pulled from dist.gem5.org.

There is no reason to keep these here. They are therefore being
removed.

Change-Id: I65d1237e275da4df6026090d8a064f47ada09687
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33140
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>